Enkephalin. Computer graphic representation of a molecule of enkephalin, an opioid peptide found in the human brain. It has a painkilling effect, and is thought to affect mood & stimulate motivation. Enkephalin is a small protein molecule made up of five peptides (pentapeptide). The molecule is depicted consisting of spherical atoms, colour- coded: carbon (blue), hydrogen (yellow), oxygen (green), nitrogen (pink). Naturally produced in the brain and by nerve endings elsewhere in the body, its analgesic (painkilling) effects resemble morphine and codeine. Enkephalin was thought to be an endorphin, but it differs chemically.